Bulk materials, such as ores, minerals, metal chips and other particulates, are often delivered to transport vehicles or deposited in stockpiles with the use of various types of mechanical conveyors. One such type of mechanical conveyor is the drag conveyor.
A drag conveyor consists of a rectangular U-shaped outer casing and an inner heavy duty single or double strand continuous chain assembly having plow blades or panels, sometimes known as drag flights, rigidly secured thereto for movement with the chain relative to the outer casing. The continuous chain assembly is driven by a mechanical drive sprocket which forms part of a powered mechanical drive unit. Idler sprockets also are provided, thereby permitting continuous forward movement of the chain and plow assembly and return or recycling thereof in a continuous loop.
Such drag flight conveyors are used to convey bulk material along both horizontal and inclined paths. The bulk materials are loaded into the conveyor at one or more feed or loading stations, generally located near the beginning part of the conveyor. As such feed material is deposited within the conveyor it accumulates between plows or drag flights and is drawn forward along the bottom surface of the conveyor casing towards the discharge station of the conveyor.
One common application of such drag type conveyors is for the delivery of bulk material to waiting rail cars, truck trailers, barges, storage piles and other containers. In such applications, the conveyor is typically loaded at or near ground level or plant floor level after which the conveyor makes a transition from the horizontal, rising at an incline generally less than 90.degree. to a higher elevation at which point the conveyor path changes to horizontal once again as it continues to a terminal point over the top of transport vehicles which are to be loaded.
Discharge stations used heretofore for discharging bulk particulate materials from a drag type conveyor have included chutes of various configurations as well as fixed openings which have been cut into the conveyor bottom pan at spaced intervals therealong and slide gates fitted to the conveyor bottom and movable between either a fully closed position or a fully open position. In actual practice, none of these provides a uniform deposition of material within a waiting container, trailer or stockpile. Irregular deposition occurs and this circumstance can result in a poor load distribution within the container resulting in wasted space and lower cost effectiveness.
A disadvantage associated with the use of slide gates and chutes is the requirement that such gates and chutes be frequently repositioned to provide distribution of material over the full length of a container or stockpile. Another disadvantage with mechanical slide gates and chutes is the costly abrasive wear and maintenance associated with such repositioning.
When discharge chutes are fitted to the bottom of a drag type conveyor, the overall height of the conveyor installation must accommodate both conveyor height and chute height, resulting in a conveyor support structure or building structure of a height which is appreciably higher and more costly than needed for the loading operation.
In those installations where fixed openings of triangular, rectangular or other designs are provided in the conveyor bottom plate, very irregular discharge often occurs. This undesirable circumstance occurs because as bulk solid materials are dragged forward across openings in the conveyor bottom resulting in the gradual discharge of material, the angle of repose of such material is constantly being altered, thereby affecting its free flow discharge behavior. Owing to variations in particle size, shape and moisture content, it is impossible to select a particular pattern of fixed slot openings in the conveyor bottom which will at all times result in the desired uniform discharge. Moreover, if the same conveyor is to be used for the conveyance and discharge of markedly different types of material, such as both coal and cast iron chips, it is highly unlikely that a pattern of fixed discharge openings which might work reasonably well for one material will be suitable for uniformly unloading the second material due to the difference in free flow characteristics exhibited by the different materials.
Another major shortcoming associated with the use of an overhead drag type conveyor having a fixed number of discharge sections, each having discharge outlets having fixed dimensions, is the fact that trucks or containers of different lengths cannot be accommodated. For example, if an overhead drag conveyor is set up with three fixed discharge outlets arranged to deposit materials into trailers of only one length, such as a 28 foot long trailer, these fixed outlets would be inadequate for properly loading a longer trailer, such as a 45-50 foot long trailer.